Background. Black women with breast cancer (BC) in Nigeria have higher mortality rate compared with British women. This study investigated prognostic features of cell proliferation biomarker (Ki-67) in Nigerian breast cancer women. Materials and Methods. The protein expression of Ki-67 was investigated in series of 308 Nigerian women, prepared as a tissue microarray (TMA), using immunohistochemistry. Clinic-pathological parameters, biomarkers, and patient outcome of tumours expressing Ki-67 in Nigerian women were correlated with UK grade-matched series. Results. A significantly larger proportion of breast tumours from Nigerian women showed high Ki-67 expression. Those tumours were significantly correlated with negative expression of the steroid hormone receptors (ER and PgR), p21, p27, E-cadherin, BRCA-1, and Bcl-2 (all ), but positively associated with EGFR ( ), p53, basal cytokeratins: CK56, CK14, triple negative, and basal phenotype using Nielsen’s classification (all ) compared to UK women. Multivariate analyses showed that race was also associated with BCSS independent of tumour size, lymph node status, and ER status. Conclusion. Ki-67 expression was observed to have contributed to the difference in the BCSS in Nigerian compared with British BC women. Therefore, targeting Ki-67 in the indigenous black women with BC might improve the patient outcome in the black women with BC. 1. Introduction There are discrepancies in mortality rates among the nationalities with Caucasian women having a low mortality rate compared with black women [1–6]. In African-American women, the effect is more pronounced in younger women compared with European-American women [3, 7]. Although there are international variations in the mortality rates of breast cancer, the explanations that predispose the cells to aggressive tumour phenotype and patient survival are not clear. Cell proliferation is one of the important driving steps in patients with aggressive tumour phenotype and patient survival [8, 9]. Cell proliferation is controlled by regulatory proteins that ensure orderly progression of the cells through the check points of the cell cycle [10–12]. The cell cycle checkpoints comprise of 4 important phases which are arranged sequentially: G1 phases that prepare their machinery for duplication, S phase is responsible for the genomic materials’ duplication, G2 phase is known as intervention phase, and the M phase that controls mitosis [13]. Abnormal cell cycle regulatory protein activities are central to increase cell proliferation, poor maintenance of chromosomal integrity,
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